GCSE Biology - How to Solve: Genetic Crosses (Punnett Squares, Genotypic/Phenotypic Ratios) – Complete Guide

How to Solve: Genetic Crosses (Punnett Squares, Genotypic/Phenotypic Ratios) – Complete Guide

Target Exam: GCSE / A-Level Biology (AQA, Edexcel, OCR) Score Impact: 6–12 marks per paper (10–20% of your grade). Mastering this unlocks inheritance questions, pedigree analysis, and genetic disorders—key for top grades.

Introduction

"If you can predict whether a child will inherit cystic fibrosis or a flower will be purple, you can answer every genetic cross question on your exam—and save 10+ marks."

WHAT YOU NEED TO KNOW FIRST

1. Alleles – Different versions of the same gene (e.g., B for brown eyes, b for blue).
2. Dominant vs. Recessive – Dominant alleles (B) mask recessive ones (b).
3. Homozygous vs. Heterozygous – BB or bb (homozygous), Bb (heterozygous).

KEY TERMS & FORMULAS

Terms to Memorise

Formulas

1. Genotypic Ratio = Count of each genotype (e.g., 1 BB : 2 Bb : 1 bb).
2. MEMORISE THIS – Examiners ask for this exact format.
3. Phenotypic Ratio = Count of each phenotype (e.g., 3 brown : 1 blue).
4. MEMORISE THIS – Often worth 2 marks alone.

STEP-BY-STEP METHOD

Follow these steps for EVERY genetic cross question.

1. Identify the parents’ genotypes.
2. If given phenotypes (e.g., "brown-eyed parent"), convert to genotypes using dominance rules.

3. Example: Brown eyes (B) dominant → BB or Bb.

4. Write the gametes each parent can produce.

5. Each gamete gets one allele per gene.

6. Example: Parent Bb → gametes: B or b.

7. Draw a Punnett Square.

8. Parent 1 gametes on top.
9. Parent 2 gametes on the side.

10. Fill in the boxes by combining alleles.

11. Count the genotypes in the Punnett Square.

12. Write the genotypic ratio (e.g., 1 BB : 2 Bb : 1 bb).

13. Convert genotypes to phenotypes.

14. Use dominance rules to find the phenotypic ratio (e.g., 3 brown : 1 blue).

15. Answer the question.

16. If asked for probability, convert ratio to percentage (e.g., 2/4 Bb = 50%).

WORKED EXAMPLES

Example 1 – Basic (Monohybrid Cross)

Question: In pea plants, tall (T) is dominant to short (t). Cross a heterozygous tall plant (Tt) with a short plant (tt). What is the phenotypic ratio of the offspring?

Step 1: Parents’ genotypes = Tt × tt. Step 2: Gametes = T or t (from Tt), t (from tt). Step 3: Punnett Square:

Step 4: Genotypic ratio = 2 Tt : 2 tt → 1 Tt : 1 tt. Step 5: Phenotypic ratio = 2 tall : 2 short → 1 tall : 1 short. Step 6: Answer = 1:1 phenotypic ratio.

What we did and why: - We used the Punnett Square to predict all possible offspring. - The ratio shows equal chances of tall or short plants.

Example 2 – Medium (Dihybrid Cross)

Question: In rabbits, black fur (B) is dominant to white (b), and short fur (S) is dominant to long (s). Cross two heterozygous rabbits (BbSs × BbSs). What is the phenotypic ratio?

Step 1: Parents’ genotypes = BbSs × BbSs. Step 2: Gametes = BS, Bs, bS, bs (from each parent). Step 3: Punnett Square (16 boxes):

Step 4: Count phenotypes: - Black, short fur (B_S_) = 9 - Black, long fur (B_ss) = 3 - White, short fur (bbS_) = 3 - White, long fur (bbss) = 1

Step 5: Phenotypic ratio = 9:3:3:1.

What we did and why: - We used FOIL (First, Outer, Inner, Last) to list all gametes. - The 9:3:3:1 ratio is classic for dihybrid crosses—memorise it!

Example 3 – Exam-Style (Disguised Question)

Question: A couple has a child with cystic fibrosis (recessive, ff). Neither parent has the disease. What is the probability their next child will be a carrier (Ff)?

Step 1: Parents must be carriers (Ff × Ff) because: - Child has ff → inherited f from both parents. - Parents don’t have the disease → must be Ff.

Step 2: Gametes = F or f (from each parent). Step 3: Punnett Square:

Step 4: Genotypic ratio = 1 FF : 2 Ff : 1 ff. Step 5: Probability of carrier (Ff) = 2/4 = 50%.

What we did and why: - We deduced the parents’ genotypes from the child’s phenotype. - The 2/4 ratio means a 50% chance of being a carrier.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

"Listen up—this is your 60-second cheat sheet for genetic crosses. First, write the parents’ genotypes. If the question gives phenotypes, use dominance rules to figure them out. Next, list the gametes—remember, each gamete gets one allele per gene. Then, draw the Punnett Square and fill it in. Count the genotypes for the genotypic ratio, then convert to phenotypes for the phenotypic ratio. If they ask for probability, turn the ratio into a fraction. Watch out for hidden carriers—if a child has a recessive disease but the parents don’t, they’re both Ff. And if it’s a dihybrid cross, use FOIL to list all four gametes. That’s it—now go smash those exam questions!"